Method of producing silicon steel hot rolled sheets having excellent surface properties

ABSTRACT

In a production method for silicon steel hot rolled sheets by subjecting a slab of silicon steel to a rough hot rolling through high-temperature heating and then subjecting to a finish hot rolling, rolling at the first stand in the finish hot rolling is carried out so that a relation of thickness at entrance side of the stand t F1  (mm), thickness at delivery side thereof t F2  (mm), surface temperature of steel sheet at gripping T F0  (° C.) and temperature at the depth of (t F1  -t F2 )/2 (mm) from the surface of the steel sheet at gripping T F1  satisfies the following equation: 
     
         (T.sub.F1 -T.sub.F0)/{(t.sub.F1 -t.sub.F2)/2}≦10+t.sub.F1 /10 
    
      (°C./mm). 
     Thus, surface defect and surface cracks can be prevented in the hot rolling to provide silicon steel sheets having excellent surface properties.

TECHNICAL FIELD

This invention relates to a method of producing silicon steel hot rolledsheets, and more particularly to a method of producing silicon steel hotrolled sheets having excellent surface properties.

BACKGROUND ART

Grain-oriented magnetic steel sheets are used as a material for ironcore in transformers and other electrical machinery and apparatus andrequired to have a high magnetic flux density and a low iron loss. Thesemagnetic properties are attained by providing secondary recrystallizedstructure with a texture having {110} face in parallel to a rolling faceand <001> axis along a rolling direction or having so-called Gossorientation as a main direction.

For this purpose, various components including silicon are added to thegrain-oriented magnetic silicon steel sheet. However, it is known thatthe workability lowers and particularly surface cracks and surfacedefects are apt to be considerably produced through hot rolling. If thedegree of the surface defects is conspicuous, not only the appearance ispoor, but also the degradation of the properties such as lowering oflamination factor, lowering of interlaminar insulation property and thelike is caused. Therefore, it is an important matter how to prevent suchsurface cracks and surface defects in view of the production step.

As a method of decreasing cracks at the hot rolling step for thegrain-oriented silicon steel sheet, there have hitherto been proposed amethod of controlling intergranular oxidation by the addition of Mo orthe like as described in JP-A-61-9521, a method of decreasing cracks byrefining the structure through recrystallization as described inJP-A-2-182832, JP-A-3-115526 and JP-A-62-149815, and the like. However,these methods are not involved in drastic settlements.

Furthermore, JP-A-63-295044 proposes a method of controlling generationof slag by setting an existing time in a high-temperature furnace duringthe heating of slab to a certain upper limit, which brings about therestriction of operation to lower the productivity.

As mentioned above, the conventional techniques for preventing cracks ofsilicon steel sheet in the hot rolling do not yet provide satisfactoryresults.

DISCLOSURE OF INVENTION

It is an object of the invention to provide a method capable ofproducing silicon steel hot rolled sheets having good surface propertieswhile effectively preventing generation of surface cracks from a newviewpoint that stress condition in the rolling deformation is improvedto prevent generation of surface cracks by controlling a temperaturedistribution in the thickness direction.

The inventors have detailedly investigated a relationship between atemperature distribution in the thickness direction of a steel sheet anda state of surface cracks generated every a stand in rough and finishrolling at hot rolling step and found that the temperature distributionin the thickness direction of the steel sheet at the first stand ofrough rolling and/or finish rolling has particularly a specific relationto the generating frequency of surface cracks and the temperaturedistribution in the thickness direction of the steel sheet is renderedinto a particular range in accordance with thicknesses at entrance anddelivery sides of said stands, and as a result the invention has beenaccomplished.

The feature and construction of the invention are as follows.

A method of producing silicon steel hot rolled sheets having excellentsurface properties by subjecting a slab of silicon steel containing Si:2.0-4.5 wt % to a rough hot rolling and then subjecting to a finish hotrolling is characterized in that rolling at the first stand of the roughhot rolling is carried out so that a relation of thickness at entranceside of the stand t_(R1) (mm), thickness at delivery side thereof t_(R2)(mm), surface temperature of the steel sheet at gripping T_(R0) (° C.)and temperature at the depth of (t_(R1) -t_(R2))/2 (mm) from the surfaceof the steel sheet at gripping T_(R1) satisfies the following equation(first invention):

    (T.sub.R1 -T.sub.R0)/{(t.sub.R1 -t.sub.R2)/2}≦10 (° C./mm)

A method of producing silicon steel hot rolled sheets having excellentsurface properties by subjecting a slab of silicon steel containing Si:2.0-4.5 wt % to a rough hot rolling and then subjecting to a finish hotrolling is characterized in that rolling at the first stand in thefinish hot rolling is carried out so that a relation of thickness atentrance side of the stand t_(F1) (mm), thickness at delivery sidethereof t_(F2) (mm), surface temperature of the steel sheet at grippingT_(F0) (° C.) and temperature at a depth of (t_(F1) -t_(F2))/2 (mm) fromthe surface of the steel sheet at gripping T_(F1) satisfies thefollowing equation (second invention):

    (T.sub.F1 -T.sub.F0)/(t.sub.F1 -t.sub.F2)/2≦10+t.sub.F1 /10 (° C./mm)

A method of producing silicon steel hot rolled sheets having excellentsurface properties by subjecting a slab of silicon steel containing Si:2.0-4.5 wt % to a rough hot rolling and then subjecting to a finish hotrolling is characterized in that rolling at the first stand in the roughhot rolling is carried out so that a relation of thickness at entranceside of the stand t_(R1) (mm), thickness at delivery side thereof t_(R2)(mm), surface temperature of the steel sheet at gripping T_(R0) (° C.)and temperature at a depth of (t_(R1) -t_(R2))/2 (mm) from the surfaceof the steel sheet at gripping T_(R1) satisfies the following equation:

    (T.sub.R1 -T.sub.R0)/{(t.sub.R1 -t.sub.R2)/2} ≦10 (° C./mm)

and rolling at the first stand in the finish hot rolling is carried outso that a relation of thickness at entrance side of the stand t_(F1)(mm), thickness at delivery side thereof t_(F2) (mm), surfacetemperature of the steel sheet at gripping T_(F0) (° C.) and temperatureat a depth of (t_(F1) -t_(F2))/2 (mm) from the surface of the steelsheet at gripping T_(F1) satisfies the following equation (thirdinvention):

    (T.sub.F1 -T.sub.F0)/(t.sub.F1 -t.sub.F2)/2≦10+t.sub.F1 /10 (° C./mm)

In case of controlling the temperature distribution in the thicknessdirection of the steel sheet at the first stand of the finish hotrolling as in the second or third invention, it is desired to avoid thelowering of the surface temperature of the steel sheet as far aspossible. For this purpose, it is favorable that the steel sheet issubjected to the finish hot rolling without substantially conductingwater cooling after the rough hot rolling.

From the same reason as mentioned above, it is favorable that descalingconducted between the rough hot rolling and the finish hot rolling inthe second or third invention is carried out by water jetting at thepressure of not more than 15 kgf/cm², or by steam spraying, gas sprayingor mechanical means.

Furthermore, it is desirable to conduct a heat holding treatment betweenthe rough hot rolling and the finish hot rolling in the second or thirdinvention.

As to the method of defining the temperature distribution in thethickness direction, JP-B-4-124218 proposes a method wherein temperatureranging from the surface of the sheet to the depth corresponding to 1/5of the thickness is defined to 1200°-1250° C. at the final stand of therough rolling to provide excellent magnetic properties. This method isto improve the magnetic properties by the improvement of texture, whichcan not expect the improving effect on the surface cracks aimed at theinvention.

Furthermore, in Japanese Patent Application No. 3-163391 filed by theapplicants, there is proposed a method wherein the rough rolling isfirst carried out at not lower than 1350° C. in the region ranging froma center of the sheet to a position corresponding to 2/5 of thethickness and a final rolling pass thereof is carried out so that thetemperature in the region ranging from the center of the sheet to theposition corresponding to 2/5 of the thickness is not lower than 1250°C. and the temperature in the region ranging from the surface to aposition corresponding to 1/5 of the thickness is 1200° C. This methodis to control the precipitation of inhibitors at the layer of aspecified thickness and has no effect on the prevention of the cracks.

Moreover, JP-A-2-138418 defines the temperature distribution in thethickness direction at the heating of the slab, which is to promote thesolution of inhibitors at the region of a specified depth and does notdevelop the effect of controlling the cracks as aimed at the inventionat all.

The cause of the surface cracks and surface defects in the hot rollingto be solved by the invention is considered to be based on the followingtheory from experimental results in a rolling testing machine andanalytical results of the stress distribution.

That is, when the temperature gradient in the thickness direction in thevicinity of the surface of the steel sheet is small at gripping to eachstand of the rough hot rolling or the finish hot rolling, the sheet issubjected to compression stress in both the thickness direction and therolling direction to cause deformation. On the other hand, when thecooling at the surface is large and the temperature gradient is large,the deformation is caused by subjecting to compression stress in thethickness direction and subjecting to tensile stress in the rollingdirection, which results in generating cracks.

The mechanism of generating cracks is due to a mechanism entirelydifferent from the conventionally known intergranular embrittlement nearmelting point.

In the rough hot rolling, cracks are remarkably produced at the firststand in which the surface temperature is highest and the texture isweak. On the other hand, the temperature distribution in the thicknessdirection is equalized through the rolling on and after the secondstand, so that the generating ratio of the cracks lowers. Therefore, ithas been found that the control of the temperature distribution in thethickness direction of the steel sheet at the first stand in the roughhot rolling is most important.

Then, the same fact as in the rough hot rolling is considered even inthe finish hot rolling. In the finish hot rolling, the generating ratioof the above cracks particularly increases when the gripping temperatureat the first stand is within a range of 800°-1000° C. Although thereason is not clear, it is considered that the inhibitors precipitateinto the intergranular phase at the above temperature range to lower theintergranular strength and hence promote the occurrence of intergranularcracks, while the precipitation of the inhibitors is not conspicuous atthe temperature outside the above temperature range and the degree ofcausing cracks decreases. The cracks in such a finish hot rolling areclosely related to the temperature distribution in the thicknessdirection of the steel sheet at the entrance side of the first stand,while on and after the second stand, the equalization of temperature inthe thickness direction is promoted and the recrystallization of thetexture is caused to lower the susceptibility to the cracks. Therefore,the control of the temperature distribution in the thickness directionof the steel sheet at the entrance side of the first finish standaccording to the invention is very important in the prevention of thecracks.

Concrete methods of decreasing the temperature gradient from the surfacetoward the thickness direction according to the invention are means byreducing or rendering water flow for cooling or scale removal before thefirst rough rolling stand and/or the first finish rolling stand intosubstantially 0, means by reducing heat dissipation due to radiation,means by increasing time up to the rolling after the cooling torecuperate heat, and means by heating from exterior alone or incombination thereof.

In case of silicon steel, it is frequent to conduct the water coolingbetween the rough hot rolling and the finish hot rolling for objectsother than descaling. Because, when the finish rolling is carried out atan excessively high temperature, the coarse precipitation of inhibitorsand the degradation of texture occur, which are unfavorable in themagnetic properties. For this end, the water cooling may be carried outby arranging a water cooling device before the finish rolling, but thereis a fear that the temperature of the sheet bar surface is lowered bythe water cooling to exceed the temperature gradient from the surfacetoward the thickness direction over the range defined in the invention.In order to avoid such a fear, the sheet bar is subjected to the finishhot rolling without substantially conducting the water cooling after therough hot rolling, while the cooling may be strengthened between thestands in the finish hot rolling to control the temperature to a desiredvalue.

Furthermore, since the formation of scale containing silicon isparticularly conspicuous in the silicon steel, new scale is producedeven between the rough hot rolling and the finish hot rolling.Therefore, in order to prevent the defect resulted from the gripping ofthe scale in the finish hot rolling, it is important to conduct thedescaling between the rough hot rolling and the finish hot rolling. Asthe descaling method, jetting high-pressure water is conventionallyknown. In this method, however, a trouble of lowering the temperature ofthe sheet bar surface becomes conspicuous. Therefore, when it isdifficult to satisfy the condition expected in the invention, the objectof the invention can be attained by decreasing the pressure of the waterflow. When the pressure of water exceeds 15 kgf/cm², the cooling effectbecomes rapidly large, so that the water pressure is desirable to be notmore than 15 kgf/cm².

In order to prevent the decrease of the surface temperature of the steelsheet, even if the descaling is carried out by steam, high-pressure gas,compressed air or the like without conducting the water jettingdescaling, it is possible to effectively conduct the descaling withoutthe decrease of the surface temperature. Furthermore, these descalingmethods can eliminate water dropwise added onto the sheet bar from asurrounding equipment or the like to reduce the influence of water evenwhen the jetting is carried out in a small amount being a smalldescaling effect, whereby the decrease of the surface temperature can beprevented. Moreover, the similar effect is obtained by mechanicallycarrying out the descaling with brush or the like.

As a more effective method for preventing the decrease of the surfacetemperature of the steel sheet, there is a method wherein a heat holdingtreatment is carried out after the rough hot rolling and before thefinish hot rolling. For example, the decrease of the surface temperaturedue to radiation can be prevented by arranging a heat holding equipment,which is made from stainless steel plate lined with a heat insulatingmaterial so as to cover the sheet bar, between rough rolling mill andfinish rolling mill and passing the rough rolled sheet bar through theheat holding equipment to the finish rolling step. This effect becomeslarge when the heat holding treatment is conducted just before thefinish rolling and the equipment is arranged over a long distance.

The most effective method is a method wherein the steel sheet is heatedby induction heating, electrical radiation heating or the like toincrease the surface temperature of the steel sheet. This method becomessomewhat high in the equipment cost but provides a very stable effect.

Moreover, the aforementioned various means may be used alone or in acombination thereof.

The slab of silicon steel used as a starting material in the inventioncontains Si: 2.0-4.5 wt %. When the Si amount is less than 2.0 wt %, theelectric resistance is low, and the iron loss based on the increase ofeddy current becomes large, and the effect of decreasing cracksaccording to the invention is not clearly recognized. While, when itexceeds 4.5 wt %, brittle cracks are apt to be caused. Therefore, it iswithin a range of 2.0-4.5 wt %.

The other components are not particularly restricted, but a typicalcomponent composition as a hot rolled sheet for grain-oriented magneticsteel sheet is mentioned as follows.

The composition contains C: 0.01-0.1 wt %, Si: 2.0-4.5 wt % and Mn:0.03-0.1 wt % and contains 0.01-0.1 wt % in total of one or two of S andSe when Mns or MnSe is used as inhibitor, or Al: 0.01-0.06 wt % and N:0.003-0.01 wt % when AlN is used as inhibitor. Moreover, MnS, MnSe andAlN may be used in admixture.

As the inhibitor, Cu, Sn, Cr, Ge, Sb, Mo, Te, Bi, P and the like areadvantageously adaptable in addition to the above S, Se and Al, so thatthey may be included in a small amount thereof.

In the first and third inventions, it is important that the rolling atthe first stand in the rough hot rolling is carried out under acondition that a relation of thickness at entrance side of the standt_(R1) (mm), thickness at delivery side thereof t_(R2) (mm), surfacetemperature of the steel sheet at gripping T_(R0) (° C.) and temperatureat a depth of (t_(R1) -t_(R2))/2 (mm) from the surface of the steelsheet at gripping T_(R1) satisfies the following equation:

    (T.sub.R1 -T.sub.R0)/{(t.sub.R1 -t.sub.R2)/2}≦10 (° C./mm).

There will be described an experiment for elucidating such a conditionbelow.

A slab of silicon steel containing C: 0.03-0.08 wt %, Si: 2.0-4.5 wt %,Mn: 0.03-0.08 wt % and Se: 0.01-0.05 wt % and the balance beingsubstantially Fe and having a thickness of 160-250 mm is heated at 1420°C. for 20 minutes and subjected to a rough rolling by varying coolingcondition.

After one pass of the rough rolling, a ratio of cracks generated perunit area in an observed surface of the steel sheet (1 m²) is measuredand shown in FIG. 1 as a relation to the value of the equation (T_(R1)-T_(R0))/{(t_(R1) -t_(R2))/2} calculated from the measured results ofsurface temperature T_(R0) and temperature T_(R1) at the depth of(t_(R2) -t_(R1))/2 at gripping when the thickness at entrance side ofthe first stand in rough rolling is t_(R1) (mm) and the thickness atdelivery side of the first stand in rough rolling is t_(R2) (mm).Moreover, this equation means a temperature gradient in the vicinity ofthe surface of the steel sheet in the thickness direction thereof.

As seen from FIG. 1, when (T_(R1) -T_(R0))/{(t_(R1) -t_(R2))/2} exceeds10, the occurrence of cracks becomes conspicuous. Therefore, accordingto the invention, the rolling at the first rough rolling stand iscarried out under the condition satisfying (T_(R1) -T_(R0))/{(t_(R1)-t_(R2))/2}≦10 (° C./mm).

In the second and third inventions, it is important that the rolling atthe first stand in the finish hot rolling is carried out under acondition that a relation of thickness at entrance side of the standt_(F1) (mm), thickness at delivery side thereof t_(F2) (mm), surfacetemperature of the steel sheet at gripping T_(F0) (° C.) and temperatureat the depth of (t_(F1) -t_(F2))/2 (mm) from the surface of the steelsheet at gripping T_(F1) satisfies the following equation:

    (T.sub.F1 -T.sub.F0)/{(t.sub.F1 -t.sub.F2)/2}≦10+t.sub.F1 /10 (° C./mm).

There will be described an experiment for elucidating such a conditionbelow.

A slab of silicon steel containing C: 0.03 wt %, Si: 2.8 wt %, Mn: 0,065wt % and Se: 0.022 wt % and the balance being substantially Fe andhaving a thickness of 200 mm is heated at 1420° C. for 20 minutes,subjected to a rough rolling to a thickness of 20 mm, 40 mm or 60 mm,and then subjected to a finish rolling by varying cooling condition tochange temperature gradient variously in the vicinity of the surface ofthe steel sheet in the thickness direction thereof.

After one pass of the finish rolling, a ratio of cracks generated perunit area in an observed surface of the steel sheet (100 cm²) ismeasured and shown in FIG. 2 as a relation to the value of the equation(T_(F1) -T_(F0))/{(t_(F1) -t_(F2))/2} calculated from the measuredresults of surface temperature T_(F0) (° C.) and temperature T_(F1) atthe depth of (t_(F1) -t_(F2))/2 (mm) at gripping when the thickness atentrance side of the first stand in the finish rolling is t_(F1) (mm)and the thickness at delivery side thereof is t_(F2) (mm). Moreover,FIG. 2a shows a case that the thickness at entrance side is 20 mm, FIG.2b shows a case that the thickness at the entrance side is 40 mm andFIG. 2c shows a case that the thickness at entrance side is 60 mm.

Next, a slab of silicon steel containing C: 0,056 wt %, Si: 3.24 wt %,Mn: 0.13 wt %, Al: 0,027 wt %, N: 0,008 wt % and S: 0,007 wt % and thebalance being substantially Fe and having a thickness of 240 mm isheated at 1300° C. for 30 minutes, subjected to a rough rolling to thethickness of 20 mm, 40 mm or 60 mm, and then subjected to a finishrolling by varying cooling condition to change temperature gradientvariously in the vicinity of the surface of the steel sheet in thethickness direction thereof.

After one pass of the finish rolling, a ratio of cracks generated perunit area in an observed surface of the steel sheet (100 cm²) ismeasured and shown in FIG. 3 as the relation to the value of theequation (T_(F1) -T_(F0))/{(t_(F1) -t_(F2))/2} calculated from themeasured results of surface temperature T_(F0) (° C.) and temperatureT_(F1) at the depth of (t_(F1) -t_(F2))/2 (mm) at gripping when thethickness at entrance side of the first stand in finish rolling ist_(F1) (mm) and the thickness at delivery side thereof is t_(F2) (mm).Moreover, FIG. 3a shows a case that the thickness at entrance side is 20mm, FIG. 3b shows a case that the thickness at entrance side is 40 mmand FIG. 3c shows a case that the thickness at entrance side is 60 mm.

The experimental results shown in FIGS. 2 and 3 are summarized in FIG. 4as a relationship between the thickness at entrance side t₁ and (T_(F1)-T_(F0))/{(t_(F1) -t_(F2))/2}. AS seen from FIG. 4, the regiongenerating cracks is dependent upon the thickness at entrance side, sothat the cracks can be prevented within a range satisfying the followingequation:

    (T.sub.F1 -T.sub.F0)/{(t.sub.F1 -t.sub.F2)/2}≦10+t.sub.F1 /10 (° C./mm).

According to the invention, therefore, the rolling at the first stand ofthe finish rolling is carried out so as to satisfy the above equation.

In the actual production steps, it is not easy to measure the interiortemperature of the slab or sheet bar. However, the interior temperaturecan be evaluated by a method detailedly described in ISIJ International.vol. 31(1991) No. 6, pp571-576, whereby the temperature controlaccording to the invention can be conducted. Moreover, the surface andinterior temperatures in the invention may be selected from typicalpoints on upper and lower surfaces and in widthwise and longitudinaldirections, but it is generally desirable to use a temperature at awidthwise central portion of the upper surface more causing the cooling.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a relation between the temperature gradient inthe thickness direction of the material and the ratio of cracksgenerated at gripping at the first stan of rough hot rolling.

FIG. 2 is a graph showing a relation between the temperature gradient inthe thickness direction of the material and the ratio of cracksgenerated at gripping at the first stand of finish hot rolling, in whichFIG. 2a shows a case at the thickness entrance side is 20 mm, FIG. 2bshows a case that thickness at entrance side is 40 mm and FIG. 2c showsa case that the thickness at entrance side is 60 mm.

FIG. 3 is a graph showing a relation between the temperature gradient inthe thickness direction of the material and/the ratio of cracksgenerated at gripping at the first stand of finish hot rolling, in whichFIG. 3a shows a case that the thicknesses at entrance side is 20 mm,FIG. 3b shows a case that thickness at entrance side is 40 mm and FIG.3c shows a case that the thickness at entrance side is 60 mm.

FIG. 4 is a graph showing the results of FIGS. 2 and 3 as a relationbetween initial thickness and the limit of genera racks.

FIG. 5 is a graph showing surface state as the cracks in Exampleconducting temperature distribution control at the first stand of finishrolling as a relation to initial thickness.

BEST MODE FOR CARRYING OUT THE INVENTION Example 1

This example shows a case of conducting temperature distribution controlat the first stand of rough rolling.

A slab of silicon steel containing C: 0.03 wt %, Si: 2.8 wt %, Mn: 0.065wt % and Se: 0.022 wt % and the remainder being substantially Fe andhaving a thickness of 200 mm is heated at 1420° C. for 20 minutes,rolled to a thickness range of from 140 mm to 180 mm at the first standof rough rolling by varying temperature distribution in the thicknessdirection of the steel sheet under various water cooling and air coolingconditions and then rolled to a thickness of 50 mm at remaining standsof rough rolling, which is subjected to a finish hot rolling of 7 standsto obtain a hot rolled sheet having a thickness of 2.0 mm.

The results of cracks observed after the rolling at the first stand ofrough rolling are shown in Table 1 together with temperature conditionsof the steel sheet at this stand.

                                      TABLE 1                                     __________________________________________________________________________     ##STR1##                                                                           ##STR2##                                                                          ##STR3##                                                                          ##STR4##                                                                          ##STR5##                                                                             ##STR6##                                                                            ##STR7##                                       __________________________________________________________________________    1 1190                                                                             1370                                                                              200 140 6      0     Acceptable                                                                    Example                                         2 1240                                                                             1380                                                                              200 160 7      0     Acceptable                                                                    Example                                         3 1260                                                                             1360                                                                              200 180 10     0     Acceptable                                                                    Example                                         4 1020                                                                             1360                                                                              200 140 17     2     Comparative                                                                   Example                                         5 1100                                                                             1340                                                                              200 160 12     7     Comparative                                                                   Example                                         __________________________________________________________________________

Example 2

This example shows a case of conducting temperature distribution controlat the first stand of rough rolling.

A slab of silicon steel containing C: 0.08 wt %, Si: 3.3 wt %, Mn: 0.074wt % and Se: 0.021 wt % and the remainder being substantially Fe andhaving a thickness of 240 mm is heated at 1420° C. for 30 minutes,rolled to a thickness range of from 140 mm to 200 mm at the first standof rough rolling by varying temperature distribution in the thicknessdirection of the steel sheet under various water cooling and air coolingconditions and then rolled to a thickness of 30 mm at remaining 3 standsof rough rolling, which is subjected to a finish hot rolling of 7 standsto obtain a hot rolled sheet having a thickness of 2.6 mm.

The results of cracks observed after the rolling at the first stand ofrough rolling are shown in Table 2 together with temperature conditionsof the steel sheet at this stand.

                                      TABLE 2                                     __________________________________________________________________________     ##STR8##                                                                           ##STR9##                                                                          ##STR10##                                                                         ##STR11##                                                                         ##STR12##                                                                            ##STR13##                                                                           ##STR14##                                      __________________________________________________________________________    1 1210                                                                             1410                                                                              240 140 4      0     Acceptable                                                                    Example                                         2 1030                                                                             1330                                                                              240 170 8.6    0     Acceptable                                                                    Example                                         3 1230                                                                             1330                                                                              240 200 5      0     Acceptable                                                                    Example                                         4 960                                                                              1340                                                                              240 170 10.9   1     Comparative                                                                   Example                                         5 1130                                                                             1370                                                                              240 200 12     4     Comparative                                                                   Example                                         __________________________________________________________________________

Example 3

This example shows a case of conducting temperature distribution controlat the first stand of finish rolling.

A slab of silicon steel containing C: 0.04 wt %, Si: 3.1 wt %, Mn: 0,054wt % and Se: 0,022 wt % and the remainder being substantially Fe andhaving a thickness of 200 mm is heated at 1420° C. for 20 minutes,rolled to a thickness of 50 mm at 3 stands of rough rolling and thensubjected to water spraying (water pressure: 5 kgf/cm²) to control asurface temperature of steel sheet to 940° C. and a temperature at thedepth of 11 mm from the surface corresponding to (t_(F1) -t_(F2))/2(t_(F1) : thickness at entrance side at the first stand, t_(F2) :thickness at delivery side at the first stand) to 1050° C., which isgripped at the first stand and subjected to finish rolling of 6 standsin total to obtain a hot rolled sheet having a final thickness of 2.0mm. In this case, the thickness at delivery side of the first stand is28 mm. After the rolling, the observation of surface cracks isconducted, and hence no crack is observed.

Example 4

This example shows a case of conducting temperature distribution controlat the first stand of finish rolling.

A slab of silicon steel containing C: 0.07 wt %, Si: 3.1 wt %, Mn: 0,062wt % and Se: 0,022 wt % and the remainder being substantially Fe andhaving a thickness of 200 mm is heated at 1400° C. for 20 minutes,rolled to a thickness of 35 mm at rough rolling of 4 stands and thensubjected to water spraying (water pressure: 10 kgf/cm²) to control asurface temperature of the steel sheet to 1030° C. and a temperature atthe depth of 8 mm from the surface corresponding to (t_(F1) -t_(F2))/2(t_(F1) : thickness at entrance side at the first stand, t_(F2) :thickness at delivery side at the first stand) to 1100° C., which isgripped at the first stand and subjected to finish rolling of 6 standsin total to obtain a hot rolled sheet having a final thickness of 2.6mm. In this case, the thickness at delivery side of the first stand is19 mm. After the rolling, the observation of surface cracks isconducted, and hence no crack is observed.

As a comparative example, a slab of silicon steel containing C: 0.07 wt%, Si: 3.1 wt %, Mn: 0.062 wt % and Se: 0.022 wt % and the remainderbeing substantially Fe and having a thickness of 200 mm is heated at1400° C. for 20 minutes, rolled to a thickness of 30 mm at rough rollingof 4 stands and then subjected to a high-pressure water spraying (waterpressure: 50 kgf/cm²) to control a surface temperature of steel sheet to850° C. and a temperature at the depth of 8 mm from the surfacecorresponding to (t_(F1) -t_(F2))/2 (t_(F1) : thickness at entrance sideat the first stand, t_(F2) : thickness at delivery side at the firststand) to 970° C., which is gripped at the first stand and subjected tofinish rolling of 6 stands in total to obtain a hot rolled sheet havinga final thickness of 2.0 mm. In this case, the thickness at deliveryside of the first stand is 14 mm. After the rolling, the observation ofsurface cracks is conducted, and hence the ratio of cracks generated is7.2 cracks/cm².

The results of the above Examples 3 and 4 and comparative example areshown in FIG. 5 as a relation between thickness at entrance side t₁ and(T_(R1) -T_(R0))/{(t_(R1) -t_(R2))/2}.

Example 5

This example shows a case that finish rolling is conducted without watercooling after the rough hot rolling.

A slab of silicon steel containing C: 0.06 wt %, Si: 3.20 wt %, Mn: 0.05wt % and Se: 0.015 wt % and the remainder being substantially Fe andhaving a thickness of 200 mm is heated at 1380° C. for 20 minutes andsubjected to rough rolling of 5 stands to a thickness of 40 mm.

Then, the steel sheet is gripped into the first stand of finish rollinginstallation without being subjected to water cooling. In the grippingat the first stand, the surface temperature is 1100° C., and thetemperature at the depth of 10 mm from the surface corresponding to(t_(F1) -t_(F2))/2 (t_(F1) : thickness at entrance side of the firststand, t_(F2) : thickness at delivery side of the first stand) is 1185°C. Such a finish rolling of 7 stands in total is carried out, in whichthe cooling between the stands is conducted by water cooling of 50kgf/cm² which is higher than the usual one, to obtain a hot rolled sheethaving a final thickness of 2.4 mm. In this case, the thickness atdelivery side of the first stand is 20 mm. After the rolling, theobservation of surface cracks is conducted, and hence no crack isobserved.

Example 6

This example shows a case that descaling through steam spraying isconducted between rough hot rolling and finish rolling.

A slab of silicon steel containing C: 0.07 wt %, Si: 2.95 wt %, Mn: 0.06wt %, S: 0.02 wt %, Al: 0.024 wt % and N: 0.008 wt % and the remainderbeing substantially Fe and having a thickness of 220 mm is heated at1410° C. for 45 minutes and subjected to rough rolling of 3 stands to athickness of 60 mm. Then. the steel sheet is subjected to steam spraying(180° C., spraying pressure: 9 kgf/cm²) to conduct the descaling and tocontrol the surface temperature to 960° C. and the temperature at thedepth of 13 mm from the surface corresponding to (t_(F1) -t_(F2))/2(t_(F1) : thickness at entrance side of the first stand, t_(F2) :thickness at delivery side of the first stand) to 1150° C., which isgripped into the first stand and subjected to finish rolling of 6 standsin total to obtain a hot rolled sheet having a final thickness of 2.8mm. In this case, the thickness at delivery side of the first stand is34 mm. After the rolling, the observation of surface cracks isconducted, and hence no crack is observed.

Example 7

This example shows a case that descaling through gas spraying isconducted between rough hot rolling and finish rolling.

A slab of silicon steel containing C: 0.07 wt %, Si: 2.95 wt %, Mn: 0.06wt %, S: 0.02 wt %, Al: 0.024 wt % and N: 0,008 wt % and the remainderbeing substantially Fe and having a thickness of 220 mm is heated at1410° C. for 45 minutes and subjected to rough rolling of 3 stands to athickness of 60 mm in the same manner as in Example 6. Then, the steelsheet is subjected to gas spraying (N₂ gas, 30° C., spraying pressure: 9kgf/cm²) to conduct the descaling and to control the surface temperatureto 1010° C. and the temperature at the depth of 13 mm from the surfacecorresponding to (t_(F1) -t_(F2))/2 (t_(F1) : thickness at entrance sideof the first stand, t_(F2) : thickness at delivery side of the firststand) to 1150° C., which is gripped into the first stand and subjectedto finish rolling of 6 stands in total to obtain a hot rolled sheethaving a final thickness of 2.8 mm in the same manner as in Example 6.In this case, the thickness at delivery side of the first stand is 34mm. After the rolling, the observation of surface cracks is conducted,and hence no crack is observed.

Example 8

This example shows a case that descaling through mechanical means isconducted between rough hot rolling and finish rolling.

A slab of silicon steel containing C: 0.07 wt %, Si: 2.95 wt %, Mn: 0.06wt %, S: 0.02 wt %, Al: 0.024 wt % and N: 0,008 wt % and the remainderbeing substantially Fe and having a thickness of 220 mm is heated at1410° C. for 45 minutes and subjected to rough rolling of 3 stands to athickness of 60 mm in the same manner as in Example 6. Then, the steelsheet is subjected to brushing to conduct the descaling and then grippedinto the first stand of finish rolling in which the surface temperatureis 1030° C. and the temperature at the depth of 13 mm from the surfacecorresponding to (t_(F1) -t_(F2))/2(t_(F1) : thickness at entrance sideof the first stand, t_(F2) : thickness at delivery side of the firststand) is 1160° C. Thereafter, the sheet is subjected to finish rollingof 6 stands in total to obtain a hot rolled sheet having a finalthickness of 2.8 mm in the same manner as in Example 6. In this case,the thickness at delivery side of the first stand is 34 mm. After therolling, the observation of surface cracks is conducted, and hence nocrack is observed.

Example 9

This example shows a case that heat-holding treatment is conductedbetween rough hot rolling and finish rolling.

A slab of silicon steel containing C: 0.03 wt %, Si: 2.95 wt %, Mn: 0.06wt % and Se: 0,015 wt % and the remainder being substantially Fe andhaving a thickness of 260 mm is heated at 1450° C. for 20 minutes andsubjected to rough rolling of 5 stands to a thickness of 30 mm. Thetemperature of the steel sheet after the rough rolling is 1250° C. atits surface.

Then, the steel sheet is passed through a heat holding equipmentarranged between the rough hot rolling installation and the finishrolling installation. The heat-holding equipment has a rectangular shapesurrounding the front and back surfaces of the steel sheet and both edgeportions thereof and is comprised of a heat insulating material ofporous alumina (thickness: 20 mm) lined with stainless steel (thickness:0.8 mm). The length is 60 m. Moreover, the rear surface side is arrangedso as to bury a gap of table rollers.

Subsequently, the steel sheet is gripped into the first stand of finishrolling, in which the surface temperature is 1190° C. and thetemperature at the depth of 5 mm from the surface corresponding to(t_(F1) -t_(F2))/2 (t_(F1) : thickness at entrance side of the firststand, t_(F2) : thickness at delivery side of the first stand) is 1230°C. Such a finish rolling of 6 stands in total is carried out to obtain ahot rolled sheet having a final thickness of 2.8 mm. In this case, thethickness at delivery side of the first stand is 20 mm. After therolling, the observation of surface cracks is conducted, and hence nocrack is observed.

Example 10

This example shows a case that heat treatment is conducted between roughhot rolling and finish rolling.

A slab of silicon steel containing C: 0.02 wt %, Si: 3.35 wt %, Mn: 0.09wt % and Se: 0.015 wt % and the remainder being substantially Fe andhaving a thickness of 200 mm is heated at 1440° C. for 20 minutes andsubjected to rough rolling of 3 stands to a thickness of 40 mm. Thetemperature of the steel sheet after the rough rolling is 1170° C. atits surface.

Then, the steel sheet is subjected to a heat treatment between the roughhot rolling installation and the finish rolling installation. The heattreatment is carried out through radiant heating process and the heatingcondition is 15 kW/m2 for 30 seconds.

Subsequently, the steel sheet is gripped into the first stand of finishrolling, in which the surface temperature is 1140° C. and thetemperature at the depth of 8 mm from the surface corresponding to(t_(F1) -t_(F2))/2 (t_(F1) : thickness at entrance side of the firststand, t_(F2) : thickness at delivery side of the first stand) is 1200°C. Such a finish rolling of 7 stands in total is carried out to obtain ahot rolled sheet having a final thickness of 2.2 mm. In this case, thethickness at delivery side of the first stand is 24 mm. After therolling, the observation of surface cracks is conducted, and hence nocrack is observed.

Example 11

This example shows a case of conducting temperature distribution controlat the first stand of rough rolling and the first stand of finishrolling.

A slab of silicon steel containing C: 0.04 wt %, Si: 3.20 wt %, Mn: 0.06wt % and Se: 0.022 wt % and the remainder being substantially Fe andhaving a thickness of 260 mm is heated at 1430° C. for 30 minutes,rolled to a thickness of 220 mm at the first stand of rough rolling bycontrolling a surface temperature of the steel sheet to 1340° C. and thetemperature at the depth of 20 mm from the surface corresponding to(t_(R1) -t_(R2))/2 (t₁ : thickness at entrance side of the first stand,t_(R2) : thickness at delivery side of the first stand) to 1410° C. andthen subjected to rough rolling of remaining 3 stands to a thickness of40 mm. Next, the steel sheet is subjected to water spraying (waterpressure: 5 kgf/cm²) to control a surface temperature to 980° C. and thetemperature at the depth of 10 mm from the surface corresponding to(t_(F1) -t_(F2))/2 (t_(F1) : thickness at entrance side of the firststand, t_(F2) : thickness at delivery side of the first stand) to 1080°C., which is gripped into the first stand and subjected to a finish hotrolling of 7 stands to obtain a hot rolled sheet having a thickness of2.6 mm. In this case, the thickness at delivery side of the first standis 20 mm. After the rolling, the observation of surface cracks isconducted, and hence no crack is observed.

Example 12

This example shows a case of conducting temperature distribution controlat the first stand of rough rolling and the first stand of finishrolling and conducting heat treatment between rough hot rolling andfinish rolling.

A slab of silicon steel containing C: 0.04 wt %, Si: 3.20 wt %, Mn: 0.06wt % and Se: 0.022 wt % and the remainder being substantially Fe andhaving a thickness of 260 mm is heated at 1430° C. for 30 minutes,rolled to a thickness of 220 mm at the first stand of rough rolling bycontrolling a surface temperature of the steel sheet to 1340° C. and thetemperature at the depth of 20 mm from the surface corresponding to(t_(R1) -t_(R2))/2(t_(R1) : thickness at entrance side of the firststand, t_(R2) : thickness at delivery side of the first stand) to 1410°C. and then subjected to rough rolling of remaining 3 stands to athickness of 40 mm in the same manner as in Example 11

Next, the steel sheet is subjected to high-pressure water spraying(water pressure: 50 kgf/cm²) to conduct descaling, in which the surfacetemperature is 860° C. and the temperature at the depth of 10 mm fromthe surface corresponding to (t_(F1) -t_(F2))/2 (t_(F1) : thickness atentrance side of the first stand, t_(F2) : thickness at delivery side ofthe first stand) is 1060° C. Then, the steel sheet is subjected to aheat treatment through radiant heating process under condition of 20kW/m² for 7 seconds, in which the surface temperature is 900° C. and thetemperature at the depth of 10 mm from the surface corresponding to(t_(F1) -t_(F2))/2 (t_(F1) : thickness at entrance side of the firststand, t_(F2) : thickness at delivery side of the first stand) is 1030°C. The steel sheet is gripped into the first stand of finish rollinginstallation and subjected to a finish rolling of 7 stands in total toobtain a hot rolled sheet having a thickness of 2.6 mm in the samemanner as in Example 11o In this case, the thickness at delivery side ofthe first stand is 20 mm. After the rolling, the observation of surfacecracks is conducted, and hence no crack is observed.

INDUSTRIAL APPLICABILITY

According to the invention, the temperature distribution in the vicinityof the steel sheet surface in the thickness direction thereof at thefirst stand of rough rolling and/or finish rolling is adjusted to belowered in accordance with the thicknesses at entrance and deliverysides of such stands, whereby grain-oriented silicon steels having veryexcellent surface properties can be produced without bringing about poorappearance, low lamination factor and low interlaminar insulatingpressure.

Furthermore, such an adjustment can easily be conducted by conducting nocooling between the rough hot rolling and the finish rolling, or byconducting heat-holding treatment or heat treatment.

Moreover, if there is caused a fear that the conditions defined in theinvention are not satisfied in high-pressure water spraying fordescaling in the adjustment, the descaling is conducted by low-pressurewater spraying, steam spraying or gas spraying instead of the waterspraying, or mechanical means, whereby the invention can surely berealized without causing the above inconveniences.

We claim:
 1. A method of producing silicon steel hot rolled sheetshaving excellent surface properties by subjecting a slab of siliconsteel containing Si: 2.0-4.5 wt % to a rough hot rolling through a firststand to form a steel sheet, having opposing surfaces, and thensubjecting said steel sheet to a finish hot rolling, characterized inthat rolling at said first stand in the rough hot rolling is carried outso that a relation of thickness at entrance side of said first standt_(R1) (mm), thickness at delivery side thereof t_(R2) (mm), surfacetemperature of said steel sheet at gripping T_(R0) (° C.) andtemperature at the depth of (t_(R1) -t_(R2))/2 (mm) from at least oneopposing surface of said steel sheet at gripping T_(R1) satisfies thefollowing equation:

    (T.sub.R1 -T.sub.R0)/{(t.sub.R1 -t.sub.R2)/2}≦10 (° C./mm).


2. 2. A method of producing silicon steel hot rolled sheets havingexcellent surface properties by subjecting a slab of silicon steelcontaining Si: 2.0-4.5 wt % to a rough hot rolling to form a steel sheethaving opposing surfaces, and then subjecting said steel sheet to afinish hot rolling through a first stand, characterized in that rollingat said first stand in said finish hot rolling is carried out so that arelation of thickness at entrance side of said first stand t_(F1) (mm),thickness at delivery side thereof t_(F2) (mm), surface temperature ofsaid steel sheet at gripping T_(F0) (° C.) and temperature at the depthof (t_(F1) -t_(F2))/2 (mm) from at least one opposing surface of saidsteel sheet at gripping T_(F1) satisfies the following equation:

    (T.sub.F1 -T.sub.F0)/{(t.sub.F1 -t.sub.F2)/2}≦10+t.sub.F1 /10 (° C./mm).


3. 3. A method of producing silicon steel hot rolled sheets havingexcellent surface properties by subjecting a slab of silicon steelcontaining Si: 2.0-4.5 wt % to a rough hot rolling through a first standto form a steel sheet having opposing surfaces, and then subjecting to afinish hot rolling through a first stand, characterized in that rollingat the first stand in said rough hot rolling is carried out so that arelation of thickness at entrance side of said first rough hot rollingstand t_(R1) (mm), thickness at delivery side thereof t_(R2) (mm),surface temperature of said steel sheet at gripping T_(R0) (° C.) andtemperature at the depth of (t_(R1) -t_(R2))/2 (mm) from at least oneopposing surface of said steel sheet at gripping T_(R1) satisfies thefollowing equation:

    (T.sub.R1 -T.sub.R0)/{(t.sub.R1 -t.sub.R2)/2}≦10 (° C./mm)

and rolling at said first stand in the finish hot rolling is carried outso that a relation of thickness at entrance side of said first finishhot rolling stand t_(F1) (mm), thickness at delivery side thereof t_(F2)(mm), surface temperature of said steel sheet at gripping T_(F0) (° C.)and temperature at the depth of (t_(F1) -t_(F2))/2 (mm) from at leastone opposing surface of the steel sheet at gripping T_(F1) satisfies thefollowing equation:

    (T.sub.F1 -T.sub.F0)/{(t.sub.F1 -t.sub.F2)/2}≦10+t.sub.F1 /10 (° C./mm).


4. 4. A method of producing silicon steel hot rolled sheets havingexcellent surface properties according to claim 2 or 3, wherein saidsteel sheet is subjected to the finish hot rolling without substantiallyconducting water cooling after the rough hot rolling.
 5. A method ofproducing silicon steel hot rolled sheets having excellent surfaceproperties according to claim 2 or 3, wherein descaling conductedbetween the rough hot rolling and the finish hot rolling is carried outby water jetting at the pressure of not more than 15 kgf/cm².
 6. Amethod of producing silicon steel hot rolled sheets having excellentsurface properties according to claim 2 or 3, wherein descalingconducted between the rough hot rolling and the finish hot rolling iscarried out without water jetting.
 7. A method of producing siliconsteel hot rolled sheets having excellent surface properties according toclaim 6, Wherein the descaling is conducted by steam spraying.
 8. Amethod of producing silicon steel hot rolled sheets having excellentsurface properties according to claim 6, wherein the descaling isconducted by gas spraying.
 9. A method of producing silicon steel hotrolled sheets having excellent surface properties according to claim 6,wherein the descaling is conducted by mechanical means.
 10. A method ofproducing silicon steel hot rolled sheets having excellent surfaceproperties according to claim 2 or 3, wherein heat holding treatment isconducted between the rough hot rolling and the finish hot rolling. 11.A method of producing silicon steel hot rolled sheets having excellentsurface properties according to claim 2 or 3, wherein heating treatmentis conducted between the rough hot rolling and the finish hot rolling.